| Stress relaxation experiments were carried out at 25°C and 150°C on 96.5Sn-3.5Ag eutectic solder and Sn-Ag composite solder (Sn-Ag eutectic solder with 20% volume Cu6Sn5 reinforcements incorporated by in-situ methods.) The magnitude of stress drop during relaxation depends upon the plastic shear strain imposed prior to the stress relaxation process. For sequential stress relaxation experiments that include unloading, stress drop is nearly independent of the accumulated plastic shear strain. However, for sequential stress relaxation that does not involve unloading, the stress relaxation is dependent upon the cumulative plastic shear strain. Stress relaxation behavior in shear is different than stress relaxation in tension. Creep rates extracted from the relaxation data were much faster with larger pre-strains in both eutectic Sn-Ag and composite solder joint. The stress exponent values (n) calculated from the stress relaxation test data ranged from 7 to 15 for both eutectic as well as composite solder joints suggesting that the deformation mechanism active during the observed stress relaxation may be controlled by dislocation climb recovery processes. |
